Wireless communication systems, such as the 3rd Generation (3G) of mobile telephone standards and technology, are well known. An example of such 3G standards and technology is the Universal Mobile Telecommunications System (UMTS™), developed by the 3rd Generation Partnership Project (3GPP™) (www.3gpp.org). The 3rd generation of wireless communications has generally been developed to support macro-cell mobile phone communications. Such macro cells utilise high power base stations (NodeBs in 3GPP parlance) to communicate with wireless communication units within a relatively large geographical coverage area. Typically, wireless communication units, or User Equipment (UEs) as they are often referred to in 3G parlance, communicate with a Core Network (CN) of the 3G wireless communication system via a Radio Network Subsystem (RNS). A wireless communication system typically comprises a plurality of radio network subsystems, each radio network subsystem comprising one or more cells to which UEs may attach, and thereby connect to the network. Each macro-cellular RNS further comprises a controller, in a form of a Radio Network Controller (RNC), operably coupled to the one or more Node Bs, via a so-called lub interface.
The second generation wireless communication system (2G), also known as GSM, is a well-established cellular, wireless communications technology whereby “base transceiver stations” (equivalent to the Node B's of the 3G system) and “mobile units” (user equipment) can transmit and receive voice and packet data. Several base transceiver stations are controlled by a Base Station Controller (BSC), equivalent to the RNC of 3G systems.
Communications systems and networks are developing towards a broadband and mobile system. The 3rd Generation Partnership Project has proposed a Long Term Evolution (LTE) solution, namely, an Evolved Universal Mobile Telecommunication System Territorial Radio Access Network, (E-UTRAN), for a mobile access network, and a System Architecture Evolution (SAE) solution, namely, an Evolved Packet Core (EPC), for a mobile core network. An evolved packet system (EPS) network provides only packet switching (PS) domain data access so a voice service is provided by either a packetised voice mechanism such as Voice over IP or a 2G or 3G Radio Access Network (RAN) and circuit switched (CS) domain network. User Equipment (UE) can access a CS domain core network through a 2G/3G RAN such as the GSM EDGE Radio Access Network (Enhanced Data Rate for GSM Evolution, EDGE) Radio Access Network or a Universal Mobile Telecommunication System Terrestrial Radio Access Network (Universal Mobile Telecommunication System Terrestrial Radio Access Network, UTRAN), and access the EPC through the E-UTRAN.
Some user equipments have the capability to communicate with networks of differing radio access technologies. For example, a user equipment may be capable of operating within a UTRAN and within an E-UTRAN.
Lower power (and therefore smaller coverage area) cells are a recent development within the field of wireless cellular communication systems. Such small cells are effectively communication coverage areas supported by low power base stations. The terms “picocell” and “femtocell” are often used to mean a cell with a small coverage area, with the term femtocell being more commonly used with reference to residential small cells. Herein, the term “small cell” means any cell having a small coverage area and includes “picocells” and femtocells. The low power base stations which support small cells are referred to as Access points (AP's) with the term Home Node B (HNB's) or Home Evolved Node B (HeNB) identifying femtocell access points. Each small-cell has one Access point. These small cells are intended to augment the wide area macro network and support communications to User Equipments in a restricted, for example, indoor environment. An additional benefit of small cells is that they can offload traffic from the macro network, thereby freeing up valuable macro network resources.
Typical applications for such Access Points include, by way of example, residential and commercial locations, communication ‘hotspots’, etc., whereby Access Points can be connected to a core network via, for example, the Internet using a broadband connection or the like. In this manner, femto cells can be provided in a simple, scalable deployment in specific in-building locations where, for example, network congestion at the macro-cell level may be problematic.
An example of a typical HNB for use within a 3GPP system may comprise Node-B functionality and some aspects of Radio Network Controller (RNC) functionality.
A HNB is an access point that provides a wireless interface for a user equipment connectivity. It provides a radio access network connectivity to a user equipment (UE) using the so-called luh interface to a network Access Controller, also known as a Home Node B Gateway (HNB-GW). One Access Controller (AC) can provide network connectivity of several HNB's to a core network.
In a small cell network it is known that there may be a very large number of small cells compared to the number of macro cells, with small cells often residing within or overlapping macro cells in the same geographic area.
Thus, the coverage area of a single macro cell will inevitably overlap (and encompass) a coverage area of a large number of small cells.
In a planned macro cell network, a so-called neighbour cell list is used to identify adjacent cells to each macro cell, to facilitate handover of UE communications from a “source” cell to a “target” cell. The neighbour cell list is broadcast to roaming UEs to enable the roaming UE to receive and assess the suitability of continuing a communication by transferring the communication to an adjacent (neighbour) cell. The neighbour cell list of the macro cell contains frequency and scrambling code information for all of the cells whose coverage area overlaps with the macro cell, to allow the UE to be able to receive and decode transmissions from the neighbouring cells.
In a macro cell network, the neighbour cell list is configured in the radio network controller (RNC) or BSC in the case of a GSM (2G) network. The RNC (or BSC) stores a neighbour cell list for each of the Node-Bs (or BTS's) that the RNC (or BSC) controls. The macro cell neighbour cell list is normally configured based on information provided by a cell-planning database. The cell-planning database can be informed of the geographic location of a Node-B (or BTS) and is able to return a list of other Node-B's (or BTS's) whose coverage areas are close to or overlap the identified Node-B (or BTS). A neighbour cell list for a Node-B (as configured at the RNC) is essentially a list of structures; with each structure containing a frequency and scrambling code to be used by the UE to access signals from every neighbour cell.
If it is assumed that a user equipment (UE) is participating in an active call, the UE receives measurement control messages to measure from the neighbour cell list in a radio resource control (RRC) system information message from the RNC via a node B. The UE measures the specified frequency and scrambling code to identify the best (generally closest) neighbouring macro cells to consider as potential target cells.
The system messages also instruct the UE as to what criteria should be used to trigger the sending of a measurement report (e.g. measuring a signal level and/or signal quality from a neighbouring cell that exceeds a predetermined threshold), and what information should be included in a measurement report.
The UE then monitors the specified neighbour cells, identified in a RRC measurement control message, until one of them meets the specified criteria. Once one of the neighbour cells meets the specified criteria, the UE sends a Measurement Report to the RNC.
Given the large number of small cells compared to the number of macro cells, it is not possible to ensure that all the small cells within the coverage area of a macro cell have individual and different frequencies and scrambling codes.
In a combined small cell-macro cell environment, the macro cell RNC may be unable to determine, from the measurement report received from the UE, which cell was measured. Notably, this problem does not occur in a standard planned (macro cell) network, as the system planners are able to ensure that the coverage area of each cell only overlaps with a small number of other cells.
However, there still remains a limitation in the size of GSM, UTRAN and LTE neighbour cell lists that makes it difficult to identify a target cell for handover in the case of a dense deployment of cells because of the re-use of identifying parameters such as PSC (Primary Scrambling Code) in the case of WCDMA (wide band code division multiple access).
Currently in 3G CELL_DCH mode (where a UE sends measurement reports to a Node B,) it is only possible to use the UARFCN (UTRA absolute radio frequency channel number) and PSC as identifiers of a potential open-access 3G target for measurement as well as quality levels to be measured, and to specify a maximum of 32 cells for measurement in each of intra-frequency, interfrequency, and inter-RAT classes. Open access cells permit any UE to access an Access Point (or HNB) or Node B, for example, and to receive the offered services. In contrast, closed access cells only allow access to a subscribed user.
In the case of a closed access (Closed Subscriber Group) cell and Release 9+ UEs solutions to the problem have been investigated based on broadcasting the PSC split for closed Access Points or HNBs and allowing the proximity indicator and autonomous search (see 3GPP TS 25.367) and reading of the cell Id.
Such mechanisms are not available in the case of 3G open cells (or for legacy UEs). The limitations described above mean that although standard means may be used to ensure that neighbouring small cells underneath a macro cell do not share the same PSC (in the case of UTRAN), a dense deployment means that the same PSC may be used by more than one cell neighbouring/underneath a macro cell, making the identification of the target cell difficult. For example, say that the number of PSC's for co-channel small cell neighbours is 32. Once 32 cells are exceeded, then at least one of the PSC's has to be re-used. So, if more than 32 small cells are underneath a macro cell, a source cell's RNC will have difficulty identifying a target small-cell uniquely. This is known as PSC Confusion.
Thus, there exists a need for an improved method and apparatus for identifying target (open) cells for handover in a cellular communication system that combines macro-cell and small cells.